INTRODUCTION 



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" yedes Thier erscheint ah eine Summe vitaler Einheiten, von denen jede den voUen 

 Charakter des Lebens an sich triigt." ViRCHOVV.i 



During the half-century that has elapsed since the enunciation of 

 the cell-theory by Schleiden and Schwann, in 1838-39, it has become 

 ever more clearly apparent that the key to all ultimate biological 

 problems must, in the last analysis, be sought in the cell. It was the 

 cell-theory that first brought the structure of plants and animals under 

 one point of view, by revealing their common plan of organization. 

 It was through the cell-theory that Kolliker, Remak, Nageli, and Hof- 

 meister opened the way to an understanding of the nature of embryo- 

 logical development, and the law of genetic continuity lying at the 

 basis of inheritance. It was the cell-theory again which, in the hands 

 of Goodsir, Virchow, and Max Schultze, inaugurated a new era in the 

 history of physiology and pathology, by showing that all the various 

 functions of the body, in health and in disease, are but the outward 

 expression of cell-activities. And at a still later day it was through 

 the cell-theory that Hertwig, Fol, Van Beneden, and Strasburger 

 solved the long-standing riddle of the fertilization of the Q.g^ and the 

 mechanism of hereditary transmission. No other biological generali- 

 zation, save only the theory of organic evolution, has brought so many 

 apparently diverse phenomena under a common point of view or has 

 accomplished more for the unification of knowledge. The cell-theory 

 must therefore be placed beside the evolution-theory as one of the 

 foundation stones of modern biology. 



And yet the historian of latter-day biology cannot fail to be struck 

 with the fact that these two great generalizations, nearly related as 

 they are, have been developed along widely different lines of research, 

 and have only within a very recent period met upon a common ground. 

 The theory of evolution originally grew out of the study of natural 

 history, and it took definite shape long before the ultimate structure 

 of living bodies was in any degree comprehended. The evolutionists 

 of the Lamarckian period gave little heed to the finer details of 

 internal organization. Thev were concerned mainly with the more 



^ Cellularpathologie, p. 12, 1858. 

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